1. Field of the Invention
The present invention relates to a process and a system for producing isocyanate using chlorine recycle to produce phosgene, which in turn is used in the production of the isocyanate.
2. Description of the Related Art
Isocyanates have become large-volume raw materials for addition polymers, such as polyurethanes, polyureas, and polyisocyanurates. By varying the reactants (isocyanates, polyols, poly-amines, and others) for polymer formation, a myriad of products have been developed ranging from flexible and rigid insulation foams to high modulus exterior parts to high quality coatings and abrasion-resistant elastomers. See Kirk Othmer, Encyclopedia of Chemical Technology, Fourth Edition, Volume 14, pp. 902-934 (1995).
A plant, or system, employing a commercial process for producing one widely manufactured type of isocyanate, toluene diisocyanate, is illustrated in the block flow diagram of FIG. 1. The plant is shown generally at 10. Plant 10 includes a dinitro toluene reactor as shown at 12 in FIG. 1. A first inlet feed line 14 as shown in FIG. 1 feeds toluene to the reactor. A second inlet feed line 16 feeds nitric acid to the reactor. The toluene is nitrated using the nitric acid to form dinitro-toluene (DNT). The dinitro-toluene is fed to a reduction reactor 18 through a line 20. In addition, hydrogen gas is fed to reduction reactor 18 through a line 22. The hydrogen gas reduces the nitro groups of the dinitro-toluene to amino groups to produce toluene diamine (TDA). This toluene diamine is sent to a toluene diisocyanate reactor 24 through a line 26 as shown in FIG. 1. Phosgene (COCl.sub.2 ) from a phosgene generator 28 as shown in FIG. 1 is fed to reactor 24 through a line 30. Chlorine is fed to the phosgene generator through a line 32, and carbon monoxide is fed to the phosgene generator through a line 34. The chlorine and the carbon monoxide react to form phosgene, as known in the art. This phosgene reacts with the toluene diamine to form toluene diisocyanate (TDI), which exits through a line 36, and essentially anhydrous hydrogen chloride (AHCl), which exits through a line 38.
The disposal of this anhydrous hydrogen chloride in the isocyanate process presents a problem from an environmental as well as an economic standpoint. This is because discharge of anhydrous hydrogen chloride is environmentally unsound. Moreover, because supply of hydrogen chloride so exceeds demand, it often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible.
Therefore, a need exists to develop a simple, inexpensive process for producing an isocyanate in which a by-product of this process, anhydrous hydrogen chloride, can be recycled in the process. Such a process would eliminate or minimize the problems of disposal of anhydrous hydrogen chloride currently associated with the production of isocyanates. In addition, such a process would decrease the cost of production of isocyanates, as it would convert the anhydrous hydrogen chloride by-product into a useful starting material for the isocyanate process.